Expensive, but I did the M-Audio subs like 15 years ago and never looked back. OEM fit, and wiring them up was simple. They work perfectly with the DSP system (were oem in the M5 with DSP) and are 10" dual voice coils, they pound out the bass. Maybe you can find a working set on Ebay?
Google search will give you all the relevant threads with part numbers and installation tips.
This video has the basic info. These are the early version of the subs. I have the later version that is a tiny bit shallower, and has small magnets on the bottom of the speaker that mate up with the ski pass through door, so when the door is open, it is held open by the magnets. Kinda slick.
Even if you found used M-Audio subs that are blown, with bad surrounds or damaged cones, you could have them reconed by a speaker shop for $50 each. So don't be afraid of buying damaged or worn out ones. That could end up being a huge savings